Memory devices can employ flash media to persistently store large amounts of data for a host device, such as a mobile device, a personal computer, or a server. Flash media includes “NOR flash” and “NAND flash” media. NAND-based media is typically favored for bulk data storage because it has a higher storage capacity, lower cost, and faster write speed than NOR media. The memory cells in NAND flash employ a charge storage structure, (e.g., a floating gate structure or a charge trapping structure) for storing charge to represent different data states. The cells are programmed by transferring electrons through a thin dielectric layer (e.g., a tunnel oxide) from a channel to, e.g., a floating gate or a charge trapping layer within the charge storage structure. The amount of charge stored in a memory cell represents one or more threshold voltages that are indicative of the voltage(s) required to form a conductive path within the channel, (e.g., depending on the amount of electrons stored on the floating gate or the charge trapping layer).
One drawback of flash memory and other non-volatile memory is that the threshold voltages of the individual memory cells can change as, over time, the memory device erases and writes data to the memory. For example, over multiple erase and write cycles, electrons can become trapped within the tunnel oxide of a memory cell, causing the threshold voltage(s) of the cell to gradually increase. This phenomenon, if uncorrected, can result in bit errors during a read of the data stored in the memory cell.
In some circumstances, error correcting code (ECC) techniques may be employed to detect and correct bit errors if the number of bit errors does not exceed the correction capacity of the code. Eventually, however, as more electrons are trapped within the tunnel oxide layers of more and more memory cells in a memory device, the number of memory cells with unreadable data states (e.g., due to shifted threshold voltages) may exceed the correction capacity of the ECC. When this happens, the memory controller is no longer able to efficiently or properly read out data from the affected memory regions.